1. Field of the invention
The present invention relates to a steering wheel equipped with an air bag device for impact absorption. More particularly, the present invention relates to an improvement in the operability of a horn switch formed in the steering wheel.
2. Description of the Prior Art
Conventional air bag devices are accommodated in a lower portion of the steering wheel pad. The known air bag device includes an air bag adapted to separate the upper surface of the pad into two parts upon operation thereof and thereafter to expand out of the pad from such a separated portion of the upper surface. One such steering wheel is disclosed in Japanese Utility Model Laid-open Publication No. 62-141559. The steering wheel there is equipped with an air bag device and includes a horn switch mounted on a side wall of the air bag retainer. Another air bag steering wheel, disclosed in Japanese Utility Model Laid-Open Publication No. 64-32251, includes a membrane switch insert molded on an upper wall of the pad which functions as the horn switch contact member. It has been proposed that this membrane switch be constructed of a pair of sheet-like electrode plates functioning as the contact member with a plurality of spacers interposed between the electrode plates serving to define an electrode gap.
It has also been proposed that such a membrane switch have a multilayer structure which comprises a plurality of independent plane switches overlapping each other as disclosed in Japanese patent Laid-open Publication No. 60-170121. In this membrane switch, the plane switches being operated are selected according to the degree of depression force being applied to the operation surface of the membrane switch
In the steering wheel having membrane switches, the horn is sounded by depressing the upper surface of the pad to short-circuit the electrode plates of the membrane switch.
However, because the plurality of spacers serving to define the electrode gap are interposed between the electrode plates as mentioned above, the depression force required to operate the switch differs when the pad is being depressed at a position just over the location of one of the spacers and when the pad is being depressed at a position not just over location of one of the spacers (see the dashed line, which indicates the characteristics of operation load about a single layer of the switch SW1, shown in FIG. 6). Thus, the depression force necessary to activate the horn with such a switch can vary greatly and is extremely location dependent.
Further, in the above-mentioned steering wheel, almost the whole lower surface of the membrane switch disclosed is closely supported by a rigid surface such as the upper surface of the air bag device. Accordingly, such a switch lacks elasticity in the direction perpendicular to the operational surface of the switch, that is, in the direction the depression force is to be applied. As a result, in the case where it is desired to short sound the horn by lightly tapping the upper surface of the pad, such as is done for greeting, this switch can not be closed long enough to reliably sound the horn for brief periods of time.
Finally, the above membrane switch causes additional complications when both the air bag device and the membrane switch are installed in the steering wheel. Because the pad is separated into two parts by the operation of the air bag device, the membrane switch should be installed in each of the two parts of the pad. Accordingly, it is necessary to provide wiring for each membrane switch. The required wiring is complicated. Furthermore, the wiring should not interfere with assembly or removal of the pad. Such requirements can not be met by any of the known methods.